Burning solid fuel



Jan. 17, 1956 M. K. BIRKNER BURNING SOLID FUEL 2 Sheets-Sheet 1 FiledOct. 25. 1949 M.If. Birkner INVENTOR BY Za/acm flm wozz ATTORNEYS Jan.7, 1956 M. K. BIRKNER BURNING SOLID FUEL.

2 Sheets-Sheet 2 Filed 001;. 25. 1949 Fig.3

I Invenfor: M if. D brlczzez United States Patent- BURNING SOLID FUELMax Karl Birkner, Koln-Dellbruck, Gemlany' Application October 25, 1949,Serial No. 123,515 Claims priority, application Germany- November 1,1948 i 19 Claims. (Cl. 1224) This invention relates to a method ofburning solid fuel which makes it possible to attain greater rates ofcombustion per unit area of grate than have hitherto been usual, andalso to apparatus for use in burning fuel by this method. Besides givinga higher fuel consumption the method dispenses with preliminaryreduction of the fuel to coal dust, and makes it possible to utilise theheat of the fiue gases for heating air, which the use of grate furnaceswith regenerative pre-heating of the feed water excludes. Through goodutilisation of the grate space and fire space the method is suitable foruse in very high duty boilers, and they can be made substantiallysmaller.

The invention makes use in part of known features and in part offeatures newly devised, from which eitherv alone, or in combination witheach other or with other elements new results are obtained.

Thus use is made of the known method of allowing entering fuel to fallfreely upon a grate to promote drying and ignition of the coal, as wellas of what is known as fuel fiuidisation, the maintenance of the fuelparticles upon the grate in hovering movement byv a powerful under-grateblast.

Use is made also of combustion of fuel in a statejof suspension, as hasbeen proposed for gas suspension combustion and as occurs in suspensiongas producers.

For this purpose use is made of highly heated air in manner in partalready known, but so as to attain new effects.

The new method also makes use of under-grate blast increasing towardsthe fire bridge end, such as is known in zoned travelling grates, but ina more powerful form to attain a new efiect.

Use is also made of the returning of gas-borne coke to the grate, butagain with a new efiect.

Such new effects also result from the employment of higher airtemperatures, difierent steps of air temperature, and substantiallygreater under-grate air pressures than were hitherto usual in theburning of fuel on grates or as dust.

The burning of fuel according to the new method is grate-burningcombined with burning in suspension. It is preferable to use travellinggrates or other grates which forward the fuel. Hitherto only a smallpart of such grates has been elfectively utilised. The front third ofthe length of the grate has served more for drying the fuel andgradually igniting it throughout, and as a rule the rear third is butlightly loaded in order that combustion may be complete. These twosections are missing in grates operating on the method of this inven-.

tion; the grates are intensively utilised throughout their length, evenmore intensively than is the middle third of existing grates.

The basis of the new method is the production of a' mixture of freshfuel entering thefire box with already burning particles of fuel. Tobring this about the amount of pressure of the under-grate blast beneatha'travelling grate or other grate which forwards the fuel is so much2,730,997 k Patented Jan. 17, 1956 ice 2. increased towards the end ofthe grate that burning particles of coal are lifted from the grate andflung into the combustion space. They are then projected toward thefront into the incoming fuel to fall simultaneously with the freelyfalling pre-dried fresh fuel which is strewed over the whole widthof thefront end of the grate, and interspersed with flame, so charging thegrate that vigorous combustion at once ensues.

According to'the invention combustion in'suspension is accelerated bythe use of air at a high temperature. The grate, on the other hand, maybe fed with a cold under blast. This has two advantages; the grate iswell cooled even when most intensively utilised, and the part of thetotal'air for combustion to be supplied above the grate can be heated toa far higher temperature than would be possible if the whole of the airwere pre-heated.

Combustion in suspension results in a great enrichment of the furnacefumes with fume-borne coke and ash, which are separated in the boilerfines and in a fume filter following the furnace. In the method of theinvention fume-borne coke and ash are separated and blown by hotair-over the travelling grate towards the front end of'the grate. Theconsequence of using air at high temperature is that the combustibleconstituents begin to be gasified while still within the conduit throughwhich they are blown forward; so quite different, and much better,conditions of combustion are obtained than by simple return offume-borne coke as hitherto known, which leads to a large part of theseparated coke blown in falling upon the grate still unburnt, andgenerally to a still larger part having to be repeatedly circulated withthe fumes. In the new method only the coarsest constituents of theseparated coke are flung into the fresh fuel falling upon the grate,where they contribute to increase the flame. Also the return offume-borne combustible, which may also be efiected in known manner withfurnace fumes as the carrier, gives rise to an initial turbulence in thefurnace gases, which brings about complete combustion.

Therefore this invention has for a principal object to provide a methodand apparatus for burning solid fuel in a vertically extendingcombustion zone having an up per outlet and a tire bed travelling fromfront to rear of the zone at the bottom thereof. The fire bed embodiesplural aligned portions including at least front, middle and rearportions. All fuel to be burned is continuously dropped solely bygravity upon the front portion of the fire bed from an inlet in thefront of the zone at a height substantially midway of the height of thezone. A primary oxidizing medium is introduced under pressure frombeneath the bed under all portions thereof to flow upwardly therethroughwith the pressure increasing toward the rear of the grate. An additionaloxidizing medium is supplied above the rear portion of the bed in theform of a substantially horizontal blanketing stream of medium directedtoward the front of the zone and extending throughout substantially thewidth of the zone at a level beneath the fuel inlet and under pressuresufficient to entrain and project the fuel and incandescent particleselevated from the bed across the bed to the front of the zone tointermingle the same 7 with the incoming curtain of fuel.

' plied above the grate is directed from various points upon a pointover the middle of the grate, or is directed tangentially to a circle ofgreater or less radius about a point over the middle of the grate. Theair nozzles v or tuyeres are preferably mounted-to swing, so that theygases such as is known in coal dust burning, and this greatly prolongsthe path of. the suspended particles in the traversing of which they maybe completely burnt. By arranging the nozzles in successive planesspaced one above the other to impart whirl in opposite directions theturbulence may be intensified and the total combustion greatlyincreased.

It is preferable to provide for blowing in some air above the grate, ina direction opposite to that of grate travel,; into the return flamezone beneath the fuel entry, while further air is supplied above the.grate-for producing turbulence directed centrally or tangentially as'above I described; forthus a comparatively short zone of com pletecombustion is produced which makes it possible to build a high dutyboiler of small height-J Fuel is fed in at about mid height of the firebox, in known manner over the whole width of the grate by a Redler belt,a chain conveyor or the like. The entry is screened by a wall of coolingor heat absorbing tubes to minimize radiation effects. To furtherprotect the conveyor from radiation from the fire box and to preventback firing, an additional supply of warm air is provided to flowdownwardly from above and past the end of the conveyor, and itstemperature is adjusted according. to the rate of fuel delivery bymixing cold air with it. This additional warm air also promotes dryingof the fuel and ignition and combustion of the volatiles which are setfree in quantity in the front zone of combustion.

the invention by the fuel feedingconveyor drive, the an rangement beingsuch that a decrease in the speed of the conveyor, such as occurs whenthe boiler is lightly loaded, increases the admixture of cold air;-whilewhen the conveyorstops the supply of hot-air is cut off and the currentof cold air throttled down to what is necessary for the protection ofthe fuel supply apparatus.-

In this method of burning, the whole of the residue from the combustion,save for the trifling amount in the clean fumes from the filter, iscarried away by the grate. On account of the great rise of temperaturein the firebox due to combustion with a small excess of air, the ash isto a large extent molten, and on the rear part of the grate is more orless granulated, while theVcoke-like unburnt particles are lifted by theair blast and flung into the fire space. The consequenceis an automaticsifting, in known mannenof the combustible from the incoinbustibleparticles. To ensure complete combustion under all circumstances theresidue on the grate; may be led over a doctor or stripper into acomplete combustion shaft behind the grate.

With many fuels it may be of advantage if. the slope of the grateincreases towards the rear in known manner.

The new method of burning solid fuel and the apparatus by whichit may becarried out are further explained below with reference to theaccompanying illustration of a construction of such apparatus. The gratechosen for illustration in this example of construction is a zonedtravelling grate. I 7 7 Figure l is a .view partly in longitudinalsection and partly in elevation of aboiler installation embodying theinvention.

Figure 2 is a fragmentary view partly in perspective and partly insection diagrammatically illustrating the control of the valves thatgovern the flow of air past the fuel forwarding conveyor.

4; views one reduced scale taken along lines 4 -4 and 5-5 respectivelyof Figure 1 and at different levels in the combustion zone.

As shown in Figure 1, the boiler for burning solid fuel is constructedof suitably formed wall means that define a combustion zone and includea front wall 1 anda rear wall 2. The combustion zone-is verticallyextended and includes an upperoutlet through which fumes and products ofcombustion flow'past the boiler tubes, thence over a bafile' 3 anddownwardly through a conduit 4 from whence they change their directionand are drawn out through the stack 5 by a fan 6 after passingthrough afume filter means 7. At the'bottom of the'combustio'n zone is atravelling grate 8. Thegrate includes plural aligned portions includinga front portion or zone b on which is initiated intensive combustion, amiddle portion or zone c that has greater longitudinal extent than thefront. portion and upon whichmain, combustion occurs and a comparativelyshort rear and separating portionor zone at from which unburned fuelparticles are separated from slag and flung upwardly into the lowerportion of the combustion zone; Through omission of the hitherto usualfront drying and igniting zone and of the rear com plete combustion zonethe grate is made substantially shorter; hence there is no need to guideits return length, and since the return length or run of the grate hangsfreely, grate-tensioning means arenot required. This makes the gratemore reliable in working. At the end of the grate 'there may be a doctorblader or scraper 9 whichfa'cilitates the enclosure of the grate anddirects slag to a combustion completing or slag shaft 10. This shaft isof advantage when burning fuels of high ash content, for 'itfrnay thenhappen that particles of coke adhere to burntout slag and might pass outunconsumed with the slag. The vertically extending combustion zoneextends from the top of the grate to the point above the uppermost bankof tuyeres referred to. hereinafter. is. provided with a fuel feedopening 11 ata height substantially midway of the height of thecombustion zone. This opening extends substantially coextensive in widthwith thewidth of the zone. Operatively associated with this. opening arefuel feed means including a' bunker 12 which supplies fuel to an endlessconveyor 13 and.

includes a delivery-end which forwards all fuel to drop down through theopening 11 in the form of a falling curtain of fuel of narrow extent inrelation to the length of the fire bed or travelling upon the frontportion of the grate. Between the delivery end of the conveyor and theadjacent portion, of the front wall 1 of the fire box is a Wall ofrelatively cold tubes 14. These tubes constitute heat absorbing meansfor minimizing radiation effects on the conveyor. Above this deliveryend of the conveyor are nozzle means 15. which direct air downwardlypastthe delivery end of the conveyor and through the opening 11' withthe fuel. This downwardly directed current of air, supplied and.controlled as hereinafter set forth, also minimizes radiation effect ordamage, prevents backfire of fuel through the opening and constitutes afurther supply of oxidizing medium.

Figure 3 is a fragmentary cross sectionalview of the firebox on areduced scale and taken along lines 3 3 of The inner walls of the firebox or combustion'chamber are clad with. the cold tube system of theboiler;

Means are provided for establishing an under-grate blast beneath the.grate including a fan 16 which draws in whatcan be termed ambient orrelatively cold air, and

directs it through, a conduit 17 to a plurality of damper andtoelevateifine incandescent particles therefrom and the pressure of theair supplied beneath the rear portion d of the grate is still higherthan that pressure required'for fiuidizatifon so as to'pick up and flinglarger incandescent The front Wall 1 j N... new-Q. a.

particles upwardly from the grate. The under-grate supply of airconstitutes the supply of the primary oxidizing means. The fan 16 alsodelivers through a conduit 21 which extends up to an air heater 22mounted in the downwardly extending portion 4 of the flue or stack. Thisair heater is of conventional construction and the air after passingthrough the air heater enters the conduit 25 from which it is dischargedthrough the superposed banks of tuyeres or nozzles 26, 27 and 28referred to hereinafter. Hot air from the air heater is also directedthrough conduit 29 that feeds the nozzle means 15. A conduit 30communicates between the conduit means 17 and the conduit 29 so that theair supplied to the nozzle means 15 is a mixture of the cold aid flowingthrough the conduit 30 and the hot air flowing through conduit 29. Thehot air conduit 25 further communicates with a bank of nozzles 31 whichas shown in Figure 3, are arranged in the rear wall 2 of the fire boxportion of the boiler and directed toward the front wall. The nozzles 31project a horizontal stream of hot oxidizing medium across the grate ata level below the fuel inlet openingll. As indicated in Figure 3, thenozzles extend across the back wall 2 of the fire box so that thehorizontal blanketing stream of oxidizing medium extends throughoutsubstantially the width of the combustion zone. This medium is suppliedunder pressure so that it impinges upon the particles of fuel andincandescent particles that have been lifted upwardly from the fire bedby the under-grate blast particularly at the rear portion of thecombustion zone and projects these particles toward and into the curtainof incoming fuel.

Beneath the bank of nozzles 31 is another bank of nozzles 32 and thesenozzles are fed by an elongated conduit means 33 suitably communicatingwith the discharge side of a fan 34. A conduit 35 communicates with theinlet side of the fan at one end and with the lower portion ofdownwardly extending hot air conduit 25 at its other end. Thus hot airis drawn through conduit 35 and forced by the fan through the elongatedconduit means 33. The fume-borne coke and ash that deposits in theboiler dues and are entrained in the fume filter means 7, settle intothe lower portion of the fume filter means as at 36 and pass throughnozzles'37 which provide communication between the fume filter means andthe conduit means 33. The hot carrier air stream delivered by the fan 34picks up separated coke and ash since the nozzles 37 work injectorfashion and project the same through the nozzle means 32 in the lowerpart of the rear wall 2 of the fire box and thus blow gas-borne coke andash toward the inlet end of the grate. Since conduit means 33 iselongated and is fed by a hot air stream, the coke entrained therein isat least partially gasified before it is discharged into the combustionzone so as to produce an initial turbulence in the gases within thecombustion zone to enhance complete combustion. Thus the gas-borne cokeand ash return nozzle means are arranged in the rear wall of the tubebelow the nozzle means 31 which constitute the secondary air supplynozzle means. All of these nozzle means are supplied with air at hightemperature and are directed toward the front end of the grate and thustoward the incoming curtain of fuel.

Higher up, above this secondary air supply is a group, or a plurality ofgroups one above the other, of nozzles s1, s2, s3, s4 (Figure 4) ii, 12,t3, t4, (Figure and a similar group of nozzles embodied in the bank oftuyeres or nozzles 28. These nozzles and those illustrated in Figures 4and 5 constitute a third air supply. The jets from these nozzles aredirected from different sidespreferably from the corners of the fireboxtowards a single point, namely the point at which the vertical axisof the fire box intersects the plane of the jets. The nozzles for thethird air supply, which are also fed with air at high temperatures, arepreferably mounted on pivot means indicated diagrammatically at 38 and39, Figures 4 and 5, so that they can swing in a horizontal plane. Thismakes it possible to adjust the jets to directions tangential to acircle of greater or less radius having its centre on the axis of thefire box,- and thus produce the known whirl in the furnace gases. Thesetting of the nozzles may be such that the direction of whirl producedby one group is opposite to that produced by the next above it. i

As stated, to shield the fuel supply apparatus from radiation from thefire box and to prevent back firing of the fuel towards the bunker, adownwardly directed stream of air is produced by nozzles 15 behind thewall of heat absorbing tubes 14. This air may be preheated though not tothe highest temperature. Its temperature may be the higher the poorerthe fuel is in volatiles. The temperature of this air stream is adjustedby mixing cold air with hot air coming from the air heater. So long asthe conveyor or fuel forwarding device is running; the highesttemperature permissible for the fuel may be maintained without damage tothe structure. But when the fuel supply is much reduced the temperatureof the protecting stream of air should be reduced in dependence on thespeed of the conveyor. This can be arranged in simple fashion by using athermostat to operate throttle valves in the hot and cold air pipes.

Upon the supply of fuel being wholly interrupted the hot air supply isautomatically shut 01f altogether and at the same time the amount of airsupplied is reduced to a minimum. The conveyor 13 is driven by anelectric motor M. This motor M also drives a tachometer device T whichcontrols valves 29 and 30 in the respective hot and cold air pipes 29and 30. Such valve control means are known and operate in accordancewith the speed of movement of the conveyor and so control the valvesthat when the conveyor slows down the valve 3% is opened wider toincrease the amount of cold air that is fed to the nozzles 15 while whenthe conveyor stops, the control impulse in the tachometer device is suchthat the valve 29' is closed to shut oif the hot air and the valve 30'is turned to a position where a reduced supply of cold air enters andflows through the nozzle means 15. The details of the controls for thevalves 29' and 30' form no part of the present invention. The speed ofthe motor that drives the conveyor and thus the speed of the conveyormovement and rate of fuel delivery as stated previously, can becontrolled in accordance with the load on the boiler by conventionalmotor controls.

The arrangement shown diagrammatically in Figure 2 illustrates valve 29closed, or crosswise of duct 29, to shut oil the hot air completely;valve 30 is also crosswise of its duct 30, but as shown is truncated toallow a certain minimum cold air flow. When coil W is energized, itsarmature is drawn in against a coil spring to open valve 29. The othercoil W has an armature formed with rack teeth engaging a toothed sectoron the shaft of valve 30, and when energized pulls down the armature toopen this valve (from its most nearly closed condition) also against acoil spring. The degree of opening will be governed by the currentstrength in this coil.

In Figure 2 the conveyor is presumed to be stationary, and needle N oftachometer T in its zero position, so neither coil is energized, and thevalves have the positions shown. At low conveyor speeds needle N willmove clockwise and complete a circuit from battery B to a sector-shapedcontact C and needle N to coil W, opening valve 29. Also, from contact Candneedle N a circuit is completed to the sectorresistor R and coil W toopen valve 30'. The faster the conveyor moves, the more of resistance Ris included in this circuit, so that the current in coil W falls off toallow valve 30' to close more and more, resulting in less and less coldair being admitted via duct 30. As the conveyor slows down, theeffective resistance of R drops, coil W becomes stronger, and valve 30'opens to admit more cold air and lower the temperature of the mixture.When the conveyor stops needle N is out of contact with bar C andresistance R, so valve 29' is closed to shut off the hot air completelyand valve 30' is as fully closed as possible to allow only the minimumof cold air to enter.

The operation and working of the boiler and furnace installation abovedescribed are as follows:

To set the boiler going the grate is charged with firelighting material,or gas or oil burners provided for fire lighting are lit. By these meansthe fuel delivered by the forwarding device to the front end of thegrate is ignited so that upon the grate being started up its two frontsections are soon charged with a burning body of coal. The under'grateblast in the middle section is now so much strengthened thatfluidisation or pneumatic stirring of fuel'begins. As soon as the rearsection of the grate is charged the air pressure upon this section isslowly increased beyond the level for fluidisation of the fuel, and atthe same time the separated coke and ash return and the secondary airsupply are set going, and soon after that the third and fourth airsupplies are turned on. i

'Because of the increase in air supply and pressure from front to backin the three sectionsof the grate a flight of sparks or incandescentgranules sets in from the back to the front of the grate. This back floweffect is known, but in the conditions of operation according to theinvention it is much increased. The eifect is further greatly increasedby the action of the gas-borne dust return, and the similarly directedsecondary air supply. So from this time on incandescent particles willbe flung on the front end of the grate together with the fresh fuel, andthe grate will be charged witha mixture of fresh'coal and incandescentmaterial. This mixture is so fanned by the blast from beneath the gratethat it at once begins to burn in vigorous fashion; so there is vigorouscombustion even at the front end of the grate, of the bed of coalvresting on the first section of the grate.

Under the action of the highly heated air, at a temperature of about 350to 450 C., extremely vigorous com bustion occurs in the lower zone ofsuspension burning both on the grate and in 'the suspension. The lighterparticles of fuel which are carried into the upper complete-combustionzone of suspension burning are burnt to a very large extent by the thirdsupply of air in the course of traversing the long path along which theincoming air drives them. Because the grate is fed with cold air, and atmuch higher pressures than hitherto, it surfers hardly any wear. siblefor anything to fall through the grate.

The fresh fuel is already dried and in part freed of volatiles byradiation from the very high temperature of the fire box while it isfalling freely. The volatiles are fourth air supply completes this dustburning imme-- diately above the front end of the grate and close to thebody of fresh and incandescent fuel upon the grate. Thus it helpsfurther to raise the capacity of the grate.

The capacity of a boiler and furnace built as above described andoperated according to the method of the invention is considerablygreater than that of known boilers employing grate burning or coal dustburning; The fire box can be of much less eight, so the boiler hasnot'to be so high, and the total cost of a boiler installation,including a boiler house of reduced height, is greatly lessened.Notwithstanding the use of considerably higher air pressures and higherspeeds of flue gases, which cause great'loss of draught the operation onthe whole is much more economical. There is no great expenditure ofenorgy in grinding fuel as in coal dust burning, nor any wear "ofgrinding mills. Only very large coal need be broken up beforehand; savefor this fuels may be burnt Also it is practically impos- 8 asthey-are,'moist o'r'dry'. 'It'is not necessary to damp" too dry coal asusually required in grate burning, so there is no loss of heat inlvapourising of the added water.

Neither the 'wa'ter-tube-clad fire box nor the grate operating under themost favourable conditions .is subject to wear.

result in high efficiency. Complete combustion at the highesttemperature makes an advantageous change in the fume-borne ash, and thistogether with the return of fume-borne ash and the high gas speedsprevents the much feared sooting-up of after heating surfaces andmakesit possible to operate the boiler for very long periods. a

'It is therefore clear that the present invention pro ing preferablycold air under pressure from beneath the grate to flow upwardlytherethrough, the area beneath the grate being divided into at leastfrom, middle and rear portions with the air supplied to these respectiveportions being of progressively increasing pressure from front to rear.The pressure beneath the middle portion of the grate fiuidizing orstirring the fuel upon the grate while a greater pressure is suppledfrom beneath the rear portion of the grate, elevates incandescentparticles of fuel above the fire bed. A secondary oxidizing medium inthe form of hot air is supplied through a bank of nozzles associatedwith the rear wall and directed toward the front wall at a level belowthe fuel inlet opening. Additionally,

nozzle means are located below the last'mentioned nozzle means forreturning separated coke and ash through' the combustion zone. Thus thesecondary air supply means provides a horizontal blanketing stream thatimpinges upon and flings elevated incandescent particle's into thefalling fuel. Above/the level of the feed opening are superposed banksof tuyeres or nozzles which preferably are directed into thecombustionzone tangentially of a circle about a common point lying above themiddle of the .fire bed. These banks or nozzles elongate the flame pathto ensure complete combustion. The oxidizing medium supplied throughthese last mentioned banks of nozzles is hot since it is air coming froman air heater located in the flue. A fourth air supply includes-nozzlemeans located above the delivery end of the conveyor anddirected'downwardly to project a current of air past the delivery endand through the fuel opening together with I the fuel. This current ofair constitutes warm air supplied from a hot. air conduit into which acold air conduit communicates. The respective conduits are valvecontrolled with the control means for the valve coordinated with thedrive for the conveyor so that when the conveyor slows down and thus therate of fuel delivery decreases, the amount of cold air is increased,whereas upon cessation of movement of the conveyor the valve in thehotair conduit is closed and the valve in the cold air conduit moves toa position where only a reduced quantity of cold air is forced throughthe nozzle means above the conveyor.

It is clear therefore that the arrangement provides for.

the construction of a highly efficient boiler for burning solid fuelwhich can be of reduced vertical dimensions compared with knownconstructions which includes plural oxidizing medium supply means socoordinated and arranged relative to the introduction of medium into thecombustion zone as to facilitate combustion.

'I claim:

.1. A boiler for burning solid fuel comprising wall means including'front'and rear walls defining a vertically Good combustion of the gaseswith small excess" of air and lower loss of combustible matter in theash aven er extending combustion zone, a travelling grate movingfrom-front to rear within the confines of said zone, fuel feed meansconsisting of a conveyor above the level of said grate and disposedsubstantially midway the height of said zone, said front wall having anopening adjacent said conveyor, said conveyor including a delivery endforwarding all fuel through said opening to fall freely into said zoneand upon said grate at the front portion thereof in a falling curtain offuel of narrow extent with relation to the direction of grate travel,heat absorbing means between the delivery end of said conveyor and theadjacent portion of the front wall for minimizing radiation effects onthe conveyor, means for supplying oxidizing medium under pressure toflow from beneath said grate upwardly therethrough at a pressureincreasing toward the rear of said grate with the greatest pressureexisting beneath the rear of the grate so as to lift fuel particlesabove the rear of the grate and means for supplying oxidizing mediumunder pressure to flow through the rear wall in a direction toward thefront Wall to project lifted fuel particles into the falling curtain offuel, said last named means being below the level of the grate andbeneath the level of the opening in the front wall.

2. A boiler for burning solid fuel including walls defining a verticallyextending combustion zone, and incorporating front and rear walls, atravelling grate moving from front to rear within the confines of saidzone, fuel feed means consisting of a conveyor above the level of saidgrate, operatively associated with the front wall substantially midwaythe height of said zone and extending substantially across the width ofsaid zone, said front wall having an opening adjacent and coextensive inwidth with the width of said conveyor, said conveyor including adelivery end for forwarding all fuel to fall freely into said zonethroughout substantially the width of the zone and upon said grate atthe front portion thereof in a falling curtain of fuel of narrow extentwith relation to the direction of grate travel, means for supplyingoxidizing medium under pressure to flow from beneath said grate upwardlytherethrough at a pressure increasing toward the rear of said grate withthe greatest pressure existing beneath the rear of the grate to liftfuel particles above the rear of the grate, means for supplyingoxidizing medium under pressure to flow through the rear wall in adirection toward the front wall to project lifted fuel particles intothe falling curtain of fuel, said last named means being above the levelof the grate and beneath the level of the opening in the front wall, andmeans for supplying downwardly directed air from above the conveyor andthrough said opening past the delivery end of the conveyor and thusadjacent the zone at which said conveyor delivers fuel to the combustionzone so that the air enters the combustion zone with the fuel and insubstantiallythe same direction of flow so as to minimize radiationeffects on the conveyor, prevent backfiring of fuel through the openingin the front wall and to constitute a further supply of oxidizingmedium.

3. A boiler for burning solid fuel including walls defining a verticallyextending combustion zone, and incorporating front and rear walls, atravelling grate moving from front to rear within the confines of saidzone, fuel feed means consisting of a conveyor above the level of saidgrate, operatively associated with the front wall substantially midwaythe height of said zone and extending substantially across the width ofsaid zone, said front wall having an opening adjacent and coextensive inwidth with the width of said conveyor, said conveyor including adelivery end for forwarding all fuel to fall freely into said zonethroughout substantially the width of the zone and upon said grate atthe front portion thereof in 'a falling curtain of fuel of narrow extentwith relation to the direction of grate travel, means for supplyingoxidizing medium under pressure from beneath said grate to move upwardlytherethrough at a pressure increasing toward the rear of said grate withthe greatest pressure existing beneath the rear of the grate to liftfuel particles above the rear of the grate, means for supplyingoxidizing medium under pressure to flew through the rear Wall in adirection toward the front wall to project lifted fuel particles intothe falling curtain of fuel, said last named means being above the levelof the grate and beneath the level of the opening in the front wall, andmeans for supplying downwardly directed air from above the conveyor andthrough said opening past the delivery end of the conveyor and thusadjacent the zone at which said conveyor delivers fuel to the combustionzone so that the air enters the combustion zone with the fuel and insubstantially the same direction of flow so as to minimize radiationeffects on the conveyor, prevent backfiring of fuel through the openingin the front wall and to constitute a further supply of oxidizingmedium, said last named air supply means including means for varying thetemperature of said downwardly directed supply of air automatically independence upon the speed of the conveyor.

4. A boiler for burning solid fuel including walls defining a verticallyextending combustion zone, and incorporating front and rear walls, atravelling grate moving from front to rear Within the confines of saidzone, fuel feed means consisting of a conveyor above the level of saidgrate, operatively associated with the front wall substantially midwaythe height of said zone and extending substantially across the width ofsaid zone, said front Wall having an opening adjacent and coextensive inwidth with the width of said conveyor, said conveyor including adelivery end for forwarding all fuel to fall freely into said zonethroughout substantially the width of the zone and upon said grate atthe front portion thereof in a falling curtain of fuel of narrow extentwith relation to the direction of grate ravel, means for supplyingoxidizing medium under pressure from beneath said grate to move upwardlytherethrough at a pressure increasing toward the rear of said grate withthe greatest pressure existing beneath the rear of the grate to liftfuel particles above the rear of the grate, means for supplyingoxidizing medium under pressure to how through the rear wall in adirection toward the front wall to project lifted fuel particles intothe falling curtain of fuel, said last named means being above the levelof the grate and beneath the level of the opening in the front wall,means for supplying downwardly directed air from above the conveyor andthrough said opening past the delivery end of the conveyor and thusadjacent the zone at which said conveyor delivers fuel to the combustionzone so that the air enters the combustion zone with the fuel and insubstantially the same direction of flow so as to minimize radiationeffects on the conveyor, prevent backfiring of fuel through the openingin the front wall and to constitute a further supply of oxidizing mediumand means for varying the temperature of said downwardly directed supplyof air in dependence upon the speed of the conveyor including meansresponsive to cessation of movement of said conveyor and operative todirect a reduced supply of cold air past the delivery end of theconveyor upon cessation of movement of said conveyor.

5. A boiler for burning solid fuel including walls defining a combustionzone and incorporating spaced front and rear Walls, a travelling gratefor supporting a fire bed at the bottom of said zone and moving fromfront to rear therein, said front wall having an opening substantiallymidway of the height of the zone and extending substantially coextensivein width with the zone, fuel feed means consisting of an endlessconveyor adjacent said opening and substantially coextensive in widthwith said opening for forwarding all fuel regardless of particle size tofall by gravity, with the falling fuel providing a downwardly flowingcurtain of fuel extending substantially coextensive in width with thezone so that fuel falls upon the grate adjacent the front wall or" thezone in a restricted area of narrow extent with relation greater thanthe pressure in said front portion rainy of air esame tO'tllfi gratetravel, means dividing the area beneath the grate into at least a frontportion subjace'nt said falling fuel, a middle portion of greater longiudinal er ent than said front portion and a rear portion of less .dinalextent than said middle portion and. rear ing disposed adjacent the rearwall of said zone, r-ueass for supplying air beneath said front portion,means for supplying air beneath said middle portion at a pressure andsufficient to fluidize the fuel upon the grate at sal' portion and toelevate fine incar from, means for supplying beneat a pressure greaterthan the pressure rec, tion to pick up and fling larger incandescentparticles 1.

wardiy from the grate, pressurized means additional and hot air throughthe rear for I i zone in a hori ntal blankcting stream extending throng7 used in a out substantiahy'the width of the zone and dis substantiallyhorizontal plane above s id cor under such pressure that said hot air itrejects said incandescent particles town; curtain of falling fuel, andmeans for supplying additional air to said in vertically spaced planeabove th op ning the front \vallof said zone jets disposed forprojecting a flow within th zone in each plane, the jets in be ngdirected to provide a tangential air dot urrent to the air flow from thejets in the adjacent -6. A boiler for burning solid fuel including wallsdefining a combustion zone and incorporating spaced front and rearWalls, a travelling grate for supporting a fire bed at the bottom andsaid zone and moving fr m front to rear therein, said front Wall havingan opening substantially midway of the height of the zone and extendingsubstantially coextensive in width with the zone, fuel feed meansconsisting of arconveyor including a delivery end adjacent said openingand substantially ccextensive in width with said opening for forwardingall fuel to fall by gravity, with the falling fuel providing adownwardly flowing curtain of fuel extending substantially coextensivein width with the zone and the fuel falling upon the grate adjacent thefront wall of the zone in a restricted area of narrow extent withrelation to the direction of grate travel, means dividing the areabeneath the grate into at least a front portion subjacent said fallingfuel, a middle portion of greater longitudinal extent than said frontportion and a rear portion of less longitudinal extent than said middleportion and said rear portion being disposed adjacent the tangential airone plane rear wall of said zone, means for supplying air beneath saidf'ont portion, means for supplying air beneath said middle portion at apressure greater than the pressure in said front portion and sufficientto fluldize the fuel upon the grate superjacent said middle portion,inserts for supplying air beneath said rear portion at a pressure theopening in the front wall of said zone including a plurahty of air jetsdisposed for projecting a tangential air flow within thezone in eachplane, the jets in one plane being, directed to provide a tangential airflow counter-current to the air flow from the jets in the ad jacentplane, heat absorbing means disposed between the delivery end of theconveyor'and the adjacent portion of the front wall for minimizingradiation effects on the conveyor, means for supplying air to flow in adownward direction from above the conveyor and past the delivery end andthrough the opening in the front supplying 1 wall adjacent the zone atwhich said conveyor delivers the curtain of fuel, and fly ash returnmeans including at least one jet in therear wall of the zone beneath themeans for supplying the additional and hot air and saidjet being abovethe fire bed at the rear portion of ing substantially coextensive inwidth with the zone, fuel feed means consisting of a conveyor adjacentsaid openand substantially coextensive in Width with said opening forforwarding all fuel to fall by gravity, with the falling fuel providinga downwardly flowing curtain of fuel extending substantially coextensivein width with the zone and the fuel falling upon the grate adjacent thefrontwall of the zone in a restricted area of narrow extent withrelation to the direction of grate travel, means dividing the areabeneath the. grate into at least a front portion subjacent said fallingfuel, a middle portion ofgreater longitudinal extent than said frontportion and a rear portion of has longitudinal extent than said middleportion and said rear portion'being disposed adjacent the rear wall ofsaid zone, meansifor supplying,

primary air beneath said front portion, means for sup- .plying primaryair beneath said middle portion at a pressure greaterthan the pressurein said front portion and sufficient to fluidize the fuel upon the grateof said middle portion, means for supplying primary air beheath saidrear portion at, a pressure greater than the pressure required forfluidization to pick up and fling incandescent particles upwardly fromthe grate, means for supplying a blanketing stream of secondary and hotair through the rearwall of said zone said stream ex? tending throughoutsubstantially the width thereof in' a substantially horizontal planeabove said conveyor so that said hot secondary air impinges upon andprojects said incandescent particles toward and into the curtain,

r of falling fuel, and means for supplying tertiary air to said zone invertically spaced substantially horizontal 1 planes above the opening inthe front wall of said zone including a plurality of air jets disposedfor projecting a tangential air flow within the zone in each planeforelongating the flame path and completing combustion, the

jets in one plane being directed to provide a tangential air flowcounter-current to the air flow from the jets in the adjacent plane,fume filter means for separating i fume-borne coke and fly ash from theproducts dis char ed from said combustion zone, elongated air conduitmeans terminating in at least one nozzle in the rear wall of the zone,disposed'beneath the means for supplying secondary hot air and directedtoward the front wall, means providing communication between saidfume-filter means and the air conduit means, and means for supplying astream of hot air through said conduit means to pick up separated cokepassing from the fumefilter means to the conduit means and ash and carrythe coke and ash through the conduit means and discharge the same intothe combustion zone above the rear portion of the grate in adirection-toward the falling curtain of fuel, the combination of thelast mentioned hot air and the length of the conduit means at leastpartially gasifying the coke before it is discharged into the zone toproduce an initial turbulence in the gases withinthc substantiallymidway of the height of the zone and extending substantially coextensivein width with the zone, fuel feed means consisting of an endlessconveyor adjacent said opening and substantially coextensive in widthwith said opening for forwarding all fuel to fall by gravity, with thefalling fuel providing a downwardly flowing curtain of fuel extendingsubstantially coextensive in width with the zone and the fuel fallingonly upon the grate adjacent the front wall of the zone in a restrictedarea of narrow extent with relation to the grate travel, means dividingthe area beneath the grate into plural longitudinally aligned portions,means for supplying primary air beneath all said portions at a pressureincreasing from front to rear so that the increasing air pressure firstfluidizes the fuel upon the grate and elevates incandescent and otherparticles of fuel from the bed with larger particles being elevated atthe area adjacent the rear wall of the zone, and means for supplying apressurized horizontal blanketing stream of secondary oxidizing mediumthrough the rear wall of the zone, said stream extending throughoutsubstantially the width of the zone in a plane above said conveyor andunder such pressure that said secondary oxidizing medium impinges uponand projects the elevated particles across the zone toward and into thecurtain of falling fuel.

9. A boiler for burning solid fuel including walls defining a verticallyextending combustion zone, and incorporating front and rear walls, atravelling grate moving from front to rear within the confines of saidzone, fuel feed means consisting of a conveyor above the level of saidgrate, operatively associated with the front wall substantially midwaythe height of said zone and extending substantially across the width ofsaid zone, said front wall having an opening adjacent and coextensive inwidth with the width of said conveyor; said conveyor including adelivery end for forwarding all fuel to fall freely into said zonethroughout substantially the Width of the zone and upon said grate atthe front portion thereof in a falling curtain of fuel of narrow extentwith relation to the direction of grate travel, means for supplyingoxidizing medium under pressure beneath said grate at a pressureincreasing toward the rear of said grate, means for supplying ablanketing stream of secondary oxidizing medium extending substantiallythroughout the width of the zone and through the rear wall in asubstantially horizontal plane at a level below the opening forprojecting fuel and incandescent particles elevated from the grate bythe medium introduced beneath the some toward and into the fallingcurtain of fueL'means for supplying a tertiary supply of oxidizingmedium in vertically spaced, substantially horizontal planes above theopening including in each plane a plurality of spaced jets directedtangentially of a circle within the zone for facilitating completion ofcombustion above the opening, and means for supplying downwardlydirected air to flow from above the conveyor and through said openingand past the delivery end of the conveyor and thus adjacent the zone atwhich said conveyor delivers fuel to said combustion zone 'so that theair enters the zone with the fuel and in substantially the samedirection of flow.

10. A boiler for burning solid fuel including walls defining acombustion zone and incorporating spaced front and rear walls, atravelling grate for supporting a fire bed at the bottom of said zoneand moving from front to rear therein, said front wall having an openingtherein substantially midway of the height of the zone and extendingsubstantially coextensive in Width with the zone, fuel feed meansconsisting of an endless conveyor adjacent said opening andsubstantially coextensive in width with said opening for forwarding allfuel to fall by gravity, with the falling fuel providing a downwardlyflowing curtain of fuel extending substantially coextensive in widthwith the zone and the fuel falling only upon the grate adjacent thefront wall of the zone in a restricted area of narrow extent withrelation to the grate travel, means dividing the area beneath the grateinto plural longitudinally aligned portions, means for supplying primaryair beneath all said portions ata pressure'increasing from front to rearso that the increasing air pressure first fluidizes the fuel upon thegrate and elevates incandescentand other particles of fuel from the bedwith the larger particles being elevated at the area adjacent the rearwall of the zone, means for supplying a pressurized horizontalblanketing stream of secondary oxidizing medium through the rear wall ofthe zone, said stream extending throughout substantially the width ofthe zone in a plane beneath said conveyor and under such pressure thatsaid secondary oxidizing medium impinges .upon and projects the elevatedparticles across the zone toward and into the cur-' tain of fallingfuel, and means for supplying additional oxidizing medium in at leastone substantially horizontal plane lying above the conveyor so that theflame path is elongated and the blanketing stream of secondary oxidizingmedium at least partially separating the combustion zone into a coarsecombustion zone portion beneath the stream and a combustion completingportion above the stream.

11. A boiler as defined in and by claim 10, in which the means forsupplying the additional oxidizing medium includes at least one group ofspaced jets directed tangentially to a circle within the zone and lyingin a horizontal plane above the level of the opening. 7

12. 'A boiler as defined in and by claim 11, in which the means forsupplying additional oxidizing medium in clude plural groups of spacedjets with the jets of each group directed tangentially to a circleWithin the zone and lying in vertically spaced substantially horizontalplanes all above the level of'the opening and with'the jets of one groupdirected to provide a turbulent flow of medium counter-current to theflow from the jets of an adjacent group.

13. A method of burning solid fuel in a vertically extending combustionzone having an upper outlet and a fire bed travelling from front to rearthereof at the bottom of the zone and which fire bed embodies pluralaligned portions including at least front, middle and rear portions,said method comprising continuously dropping solely by gravity all fuelto be burnt, upon the front portion of the fire bed from an inlet in thefront of the zone at a height substantially midway of the height of thezone in the form of a falling curtain of fuel of narrow extent withrelation to the direction of travel of the bed and of a widthsubstantially coextensive in Width with the width of the zone,introducing primary oxidizing medium under pressure from beneath the bedunder all portions thereof, increasing the pressure of the mediumintroduced under the middle portion of the bed to a pressure sufiicientto fluidize and stir the fuel thereon and to elevate fine incandescentparticles therefrom, increasing the pressure of the medium introducedunder the rear portion of the bed to a pressure higher than thatrequired for fluidization and sufiicient to elevate and fling fuelparticles and larger incandescent particles upwardly from the bed, andsupplying additional oxidizing medium above the rear portion of the bedin the form of a substantially horizontal blanketing stream of mediumdirected toward the front of the zone and extending throughoutsubstantially the Width of the zone at a level beneath the fuel inletand under a pressure sufficient to entrain and project the fuel andincandescent particles elevated from the bed across the bed to the frontof the zone to intermingle the same with the incoming curtain of fuel.

14. A method of burning solid fuel in a vertically extending combustionzone having an upper outlet and a fire bed travelling from front to rearthereof at the bottom of the zone and which fire bed embodies pluralaligned portions including at least front, middle and rear portions,said method comprising continuously dropping solely by gravity all fuelto be burnt, upon the front portion of the fire bed from an inlet in thefront of the zone at a height substantially midway of the height of thezone in the form of a falling curtain of fuel of narrow extent withthereof, increasing the pressure of the medium introduced under themiddle portion of the bed to a pressure sufficient to fluidize and stirthe fuel thereon and to elevate fine incandescent particles therefrom,increasing the pressure of the medium introduced under the rear portionof the bed to pressure higher than that required for iiuidization andsuificient to elevate and fling fuel particles and larger incandescentparticles upwardly from the bed, and supplying hot additional oxidizingmedium above the rear portion of the bed in the form of a substantiallyhorizontal blanlieting' stream of medium directed toward the front ofthe zone and extending throughout substantially the width of the zone ata level beneath the fuel'inlet and under a pressure suiiicient toentrain and project the fuel and incandescent particles elevated fromthe bed across the bed to the front of the zone to intermingle the samewith the incoming curtain of fuel.

154A method of burning solid fuel in a vertically extending combustionzone having an upper outlet and a fire bed travelling from front to rearthereof at the bottom "of the zone and which fire bed embodies pluralaligned portions including at least front, middle and rear portions,said method comprising continuously dropping solely by gravity all fuelto be burnt, upon the front portion of the fire bed from an inlet in thefront of the zone at a height substantially midway of the height of thezone in the form of a falling curtain of fuel of narrow extent withrelation to the direction of travel of the bed and of a widthsubstantially coextensive in width with the width of the zone,introducing primary oxidizing medium under pressure from beneath the bedunder all portions thereof, increasing the pressure of the mediumintroduced under the middle portion of the bed to a pressure sufficientto fluidize and stir the fuel thereon and to elevate fine in width ofthe zone at a level beneath the fuel inlet and under a a pressuresufficient to entrain and project the fuel and incandescent particleselevated from the bed across the bed to the front of the zone tointermingle the same with V the incoming curtain of fuel, and to providea horizontal stream substantially throughout the zone separating thesame into a coarse combustion zone portion beneath the stream and acombustion completing portion above the stream and supplying stilladditional hot oxidizing medium above the bed and above the fuel inletfor completing combustion of any fuel in suspension and for combustionof evolved gases.

16. A method as defined in and by claim 13, further comprisingseparating fume-borne fuel particles and fly ash particles from fumesleaving the combustion zone, entraining the separated particles in anelongated stream of hot oxidizing medium, projecting the stream of hotmedium and entrained particles into the zone above the rear portion ofthe bed and in a direction toward the front of the zone at a levelbeneath said horizontal blanketing stream of oxidizing medium toinitiate turbulence above the bed and assist the projection of risingfuel and incandescent particles into the incoming curtain of fuel. V p

17. A method of burning solid fuel in a vertically extending combustionzone having an upper outlet and a fire bed travelling from front to rearthereof at the bottom of the zone and which fire bed'embodies pluralaligned .16 portions including at least front, middle and rear portions,said method comprising continuously dropping solely by, gravity all fuelto be burnt,'upon the front portion of the fire bed from an inlet in thefront of the zone at a height substantially midway of the height of thezone in the form of a falling curtain of fuel ofnarrow extent withrelation to the direction of travel of the bed, and of la widthsubstantially coextensive in width with .the width of the zone,introducing primary oxidizing medium under pressure from beneath the bedunder all portions thereof,

increasing the pressure of the medium introduced under the middleportion of the bed to a pressure sufficient to fluidize and'stir thefuel thereon and to elevate fine incandescent particles therefrom,increasing the pressure of the medium introduced under the rear portionof the bed to a pressure higher than that required for fluidizaf tionand sufiicient to elevate and fling fuel particles and largerincandescent particles upwardly from the bed,

supplying additional oxidizing medium above the rear pot:

tion of the bed in the form of a substantially horizontal 'blanketingstream of medium directed toward the front pressure'and in the form ofhorizontally directed jets and directing the jets-from spaced pointstangentially of a circle about a point over the middle portion of thebed to complete combustion of fuel, increase turbulence in the zone andthus provide an elongated fiame path;

'18. A method of burning solid fuel in a vertically extending combustionzone having an upper outlet and a fire bed travelling from front to rearthereof at the bottom of the zone and which fire bed embodies pluralaligned portions including at least front, middle and rear portions,said method comprising continuously dropping solely by gravity all fuelto be burnt, upon the front portion of the fire bed from an inlet in thefront of the zone at a height substantially midway of the height ofthezone in the form of a falling curtain of fuel of narrow extent withrelation to the direction of travel of the bed and of a Widthsubstantially coextensive in width with the width of the zone,introducing primary oxidizing medium under pressure from beneath the bedunder all portions thereof,

increasing the pressure of the medium introduced under to a pressurehigher than that required for fiuidization' and sufficient to elevateand fling fuel particles and larger incandescent particles upwardly fromthe bed, supplyw ing additional oxidizing medium above the rear portionof the bed in the form of a substantially horizontal blanketing streamof medium directed toward the front of the zone and extending throughoutsubstantially the width of the zone at a level beneath the fuel inletand under a pressure suflicient to entrain and project the fuel andincandescent particles elevated from the bed across the bed to the frontof the zone to intermingle the same with the incoming cur tain of fuel,and introducing still additional oxidizing medium above the inlet andunder pressure in the form of horizontally directed jets lying in aplurality of vertically spaced planes and directing the jets in eachplane from spaced points tangentially of a vertical cylinder about'apoint above the middle of the bed.

19. A method of burning solid fuel as claimed in claim 18, andintroducing the still'additional oxidizing medium under pressure throughthe jets of one plane in a direction counter-current to the supplyissuing from the jets in an adjacent plane.

(References on following page) References Cited in the file of thispatent UNITED STATES PATENTS Mace Sept. 2, 1902 Alford May 5, 1914 MannMar. 25, 1919 Stephens Feb. 22, 1921 Schillinger July 31, 1928 BaileyJuly 2, 1929 Coghlan et a1. Feb. 21, 1933 10 18 w Coghlan et a1 Jan. 16,1934 Stratton Aug. 14, 1934 Mosshart Oct. 9, 1945 Glaeser Oct. 4, 1949Gladden Jan. 10, 1950 FOREIGN PATENTS Australia Feb. 25, 1932 France May10, 1948

